/*

 * Licensed to the Apache Software Foundation (ASF) under one

 * or more contributor license agreements.  See the NOTICE file

 * distributed with this work for additional information

 * regarding copyright ownership.  The ASF licenses this file

 * to you under the Apache License, Version 2.0 (the

 * "License"); you may not use this file except in compliance

 * with the License.  You may obtain a copy of the License at

 *

 *     http://www.apache.org/licenses/LICENSE-2.0

 *

 * Unless required by applicable law or agreed to in writing, software

 * distributed under the License is distributed on an "AS IS" BASIS,

 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.

 * See the License for the specific language governing permissions and

 * limitations under the License.

 */

package com.bff.gaia.unified.sdk.extensions.sql.impl.transform.agg;



import com.google.auto.value.AutoValue;



import java.io.Serializable;

import java.math.BigDecimal;



/** Accumulates current covariance of a sample, means of two elements, and number of elements. */

@AutoValue

abstract class CovarianceAccumulator implements Serializable {

  static final CovarianceAccumulator EMPTY =

      newCovarianceAccumulator(BigDecimal.ZERO, BigDecimal.ZERO, BigDecimal.ZERO, BigDecimal.ZERO);



  abstract BigDecimal covariance();



  abstract BigDecimal count();



  abstract BigDecimal xavg();



  abstract BigDecimal yavg();



  static CovarianceAccumulator newCovarianceAccumulator(

      BigDecimal covariance, BigDecimal count, BigDecimal xavg, BigDecimal yavg) {



    return new AutoValue_CovarianceAccumulator(covariance, count, xavg, yavg);

  }



  static CovarianceAccumulator ofZeroElements() {

    return EMPTY;

  }



  static CovarianceAccumulator ofSingleElement(BigDecimal inputElementX, BigDecimal inputElementY) {

    return newCovarianceAccumulator(BigDecimal.ZERO, BigDecimal.ONE, inputElementX, inputElementY);

  }



  /** See {@link CovarianceFn} doc above for explanation. */

  CovarianceAccumulator combineWith(CovarianceAccumulator otherCovariance) {

    if (EMPTY.equals(this)) {

      return otherCovariance;

    }



    if (EMPTY.equals(otherCovariance)) {

      return this;

    }



    BigDecimal increment = calculateIncrement(this, otherCovariance);

    BigDecimal combinedCovariance =

        this.covariance().add(otherCovariance.covariance()).add(increment);



    return newCovarianceAccumulator(

        combinedCovariance,

        this.count().add(otherCovariance.count()),

        calculateXavg(this, otherCovariance),

        calculateYavg(this, otherCovariance));

  }



  /** Implements this part: {@code increment = (mx_A - mx_B)*(my_A - my_B)*n_A*n_B/n_X }. */

  private BigDecimal calculateIncrement(

	  CovarianceAccumulator covarA, CovarianceAccumulator covarB) {



    BigDecimal countA = covarA.count();

    BigDecimal countB = covarB.count();



    BigDecimal totalCount = countA.add(countB);



    BigDecimal avgXA = covarA.xavg();

    BigDecimal avgYA = covarA.yavg();



    BigDecimal avgXB = covarB.xavg();

    BigDecimal avgYB = covarB.yavg();



    return avgXA

        .subtract(avgXB)

        .multiply(avgYA.subtract(avgYB))

        .multiply(countA)

        .multiply(countB)

        .divide(totalCount, CovarianceFn.MATH_CTX);

  }



  /** Implements this part: {@code avg_x = (avgx_A * n_A) + (avgx_B * n_B) / n_X }. */

  private BigDecimal calculateXavg(CovarianceAccumulator covarA, CovarianceAccumulator covarB) {



    BigDecimal countA = covarA.count();

    BigDecimal countB = covarB.count();



    BigDecimal totalCount = countA.add(countB);



    BigDecimal avgXA = covarA.xavg();

    BigDecimal avgXB = covarB.xavg();



    return avgXA

        .multiply(countA)

        .add(avgXB.multiply(countB))

        .divide(totalCount, CovarianceFn.MATH_CTX);

  }



  /** Implements this part: {@code avg_y = (avgy_A * n_A) + (avgy_B * n_B) / n_Y }. */

  private BigDecimal calculateYavg(CovarianceAccumulator covarA, CovarianceAccumulator covarB) {



    BigDecimal countA = covarA.count();

    BigDecimal countB = covarB.count();



    BigDecimal totalCount = countA.add(countB);



    BigDecimal avgYA = covarA.yavg();

    BigDecimal avgYB = covarB.yavg();



    return avgYA

        .multiply(countA)

        .add(avgYB.multiply(countB))

        .divide(totalCount, CovarianceFn.MATH_CTX);

  }

}